Fiber optic sensors are popular for use in detecting changes in temperature, pressure and strain. Fiber optic sensors in which the fiber itself acts as the sensor, are of interest in the context of advanced smart structures that use composite materials in combination with metallic components. This is due to the fact that the fiber is generally compatible with both thermoset and thermoplastic composites. This makes the the fiber optic sensor particularly useful when embedded in the composite/metallic material to function as an in-situ measurement device.
In particular, the invention herein can be used in the aircraft industry where failure of aluminum components is a major concern. Current inspection techniques for such failures include manual processes that may involve disassembling parts, thereby removing the plane from service for costly extended time periods. The proposed invention herein solves such a problem by providing an on-line system for remotely monitoring the structural components that operate in a corrosive environment.
Pertinent prior art that uses the Fabry-Perot fiber optical cavity to perform physical measurements of a structural object include U.S. Pat. No. 5,202,939 by Belleville et al. entitled "Fabry-Perot Optical Sensing Device for Measuring a Physical Parameter." This patent teaches of a device that uses an optical fiber Fabry-Perot optical cavity with a Fizeau-interferometer for multimode light transduction of physical parameters such as temperature, pressure and mechanical strain. This teaching requires the use of a halogen-quartz based lamp type light source, not a single mode light source, i.e. a diode laser as in the present invention which effectively allows for corrosion measurements. Moreover, this patent teaches away from using a single mode light transmission optical fiber, which is the only mode used in the present invention. Moreover, this reference does not teach or suggest using the interferometer to measure corrosion.
U.S. Pat. No. 5,237,630 by Hogg et al. entitled "Fiber Optic Device with Reflector Located at Splice Joint" teaches of how to make fiber optic Fabry-Perot strain gauges and system for monitoring along with various ways of making the optical cavity, and how to make a Fabry-Perot reflecting surface. However, this patent does not teach or suggest the means for detecting stress-corrosion or corrosion of a structural specimen as discussed below.
Another teaching of a typical prior art Fabry-Perot strain sensors appears in the article entitled Fabry-Perot fiber optic sensors in full scale testing of the F-15 aircraft by Murphy, Applied Optics, January 1992, Vol. 31, No. 4, pages 431-433 deals exclusively with extrinsic measurements of strain and does not teach or suggest using the sensor for corrosion detection.